Heat shock protein gp96 is a key downstream chaperone in the endoplasmic reticulum (ER) that mediates ER unfolded protein responses (UPR). Cell surface expression of gp96 in a transgenic mouse leads to spontaneous lupus-like autoimmune disease which is dependent on commensal bacteria and Toll-like receptor 4 (TLR4). This finding, together with the discovery that gp96 is a master chaperone for TLRs, has moved the field forward significantly: we now hypothesize that gp96 tunes the immune system by chaperoning TLRs but not by signaling through TLRs; depending on its expression level and location, gp96 can break tolerance if upregulated and can confer immunodeficiency if compromised in expression or function. The previously unappreciated roles of TLR4 in lupus have been validated by lupus in TLR4-overexpressing mice. Studies of the mechanism of autoimmunity in gp96 transgenic mice have also uncovered the potential impact of chronic TLR hyperresponsiveness to commensal flora on IL-10-producing CD4? T cells (TR1) and IL-17-secreting CD4 cells (TH17) in vivo. In this proposal, we will critically address the hypothesis that TLR4 amplification at either DNA level or post translational level significantly impact the biology (priming, maintenance and function) of both TR1 and TH17, leading to breakdown of peripheral tolerance. We will also define the hierarchy of TLRs in driving lupus with particular focus on the roles of nucleic acid-sensing TLRs in our model. Our study is of fundamental significance in understanding the roles of TLRs in immunity and tolerance via regulating TR1 and TH17, since the dysregulation of these cell types has been increasingly implicated in autoimmune diseases. Our recent findings that the surface expression of gp96 is highly regulated in vivo and that the increased expression of gp96 on the cell surface correlates with human lupus highlight the clinical relevance of our study.